1. Field of the Invention
The present invention is directed to a wash liquid distributor for applying wash liquid uniformly across a filter cake on a filter surface, preferably a rotary filter surface, providing effective washing, the wash liquid distributor exhibiting resistance to clogging and fouling.
2. Description of the Related Art
In utilizing filters and especially rotary drum filters it is usually necessary that the solid particles deposited on the filter surfaces from the filtered liquid must be washed to remove any entrained mother liquid remaining in or on the solid deposited particles. This serves the two fold purpose of recovering additional mother liquid and producing a cleaner cake of solid deposited particles which are subsequently recovered from the filter surface.
In washing the solid deposited particles it is necessary that as fine and uniform a wash layer as possible be applied to the surface to prevent undue disturbing of the particle layer (hereinafter cake) so as to prevent uneven washing and channeling of the wash liquid. Channeling occurs when wash liquid finds a path of reduced resistance to flow in the cake and preferentially follows that path rather than being drawn evenly and uniformly through the entire cake.
Numerous arrangements and devices have been devised and suggested for evenly disposing wash liquid on filter cakes. See for instance U.S. Pat. No. 949,724, U.S. Pat. No. 1,059,327, U.S. Pat. No. 1,475,244, U.S. Pat. No. 2,092,111, U.S. Pat. No. 2,202,932, U.S. Pat. No. 2,698,687, and U.S. Pat. No. 3,215,277.
U.S. Pat. No. 3,729,414 teaches a wash distributor system wherein distribution pipes are fed with liquid from several entry points and the liquid flows outwardly through small holes in the pipes and runs down and around grooving and drips off the bottom of the pipe. In such a distributor only a very small pressure can be used since the liquid must flow around the pipe rather than jetting out of the small holes. Thus, since very little pressure can be applied to the distributor pipe in the invention, it is not possible to obtain optimum distribution along the pipe and distribution will be disturbed by changes in wash flow rates.
The process to which U.S. Pat. No. 3,729,414 was directed is a process for removal of wax crystals from lubricating oils. In that process, oil is dissolved in solvent and chilled to form wax crystals which are then removed by filtration. As will be appreciated, the wax crystals are soft and consequently the spaces between the crystals may be easily blinded by physical contact. If the wash liquid leaves the distributor pipes at a high velocity, it can impact against the cake with sufficient force to dislodge it or to cut channels in it and thereby prevent uniform washing.
Filters can also be provided with spray nozzles mounted on distributor pipes. Since spray nozzles create relatively high velocity sprays, reasonably even distribution of liquid is possible at the outlet of the spray nozzles. However, the high velocities with which the spray issues have detrimental effects on the cake porosity and are undesirable. At the same time wash rates can vary widely depending on the nature of the wax crystals. When this occurs, the shape of the spray will change with the liquid pressure and coverage by the spray nozzles will be dependent upon the wash rate. Accordingly, at low wash rates poor coverage of the cake and poor washing often occurs.
U.S. Pat. No. 4,008,154 teaches a much improved wash distribution system for use in rotary filters. Distribution occurs in the form of a multiplicity of continuous narrow pools of liquid which are laid down with a minimum disturbance of the porosity of the filter cake.
Application of the wash liquid to the cake takes place in a three-step process. First, a uniform distribution of the wash liquid along the distributor pipe is provided by using a higher than usual pressure and creating a multiplicity of jets exiting from small holes axially spaced along the pipe and facing away from the filter cake. Thereafter, the kinetic energy of the jets is damped out in a diffusion channel adjacent to the jets and extending around the exterior of the pipe.
This diffusion channel is defined as the space bounded by the exterior of the pipe and a solid cover partially surrounding the pipe and overlaying the holes. The channel contains a coarse mesh screen positioned so as to present its edge to liquid passing through said channel whereby liquid jets produced by the holes are dampened and the liquid diffuses throughout the channel. Overwrap means in the form of a helical coil of wire secure the cover to the pipe and serve to collect and redistribute the liquid exiting from the channel at a plurality of spaced drip points along the bottom of the pipe. Liquid accumulating at the drip points falls from the drip points of its own weight under the influences of gravity.
Thus it is seen that in the current art both spray nozzles and drip pipes are used to apply wash liquid to filter cake, in particular to wax cakes.
Both spray nozzles and current drip pipe designs suffer from fouling of small internal clearances by debris from upstream plant, such as products of corrosion, which are in fine particulate form, too fine to be removed economically by normal commercial strainers. The effect of fouling of spray nozzles and drip pipes is a reduction in the distribution efficiency of the wash liquid to the wax cake, leaving some parts of the wax cake unwashed or only partially washed, thereby losing filtrate yield and leaving oil in the wax cake.
It would be highly beneficial if a liquid distributor could be designed which produces an even stream of low velocity wash liquid and which does not clog or foul as a result of the presence of contaminants present in the wash liquid.